The invention relates to an anti-skid-controlled hydraulic brake system with a master brake cylinder, a pressure medium storage reservoir, at least one wheel brake connected with the master cylinder via a brake line and with the pressure medium storage reservoir via a relief line, with one pressure modulation valve at a time being inserted into the mentioned lines so that pressure application to, or pressure relief of the wheel brakes is possible in response to pressure control signals, and with a pump delivering out of the pressure medium storage reservoir and connected to the brake line via a pressure line.
A brake system of this type is described in German Published Patent Application (DE-OS) No. 36 01 914.
There are two kinds of brake actuation to be distinguished from each other, namely conventional brake application and brake-slip-controlled application. During a conventional braking operation the pump is switched off, the modulation valve in the brake line (inlet valve) is open, and the modulation valve in the relief line (outlet valve) is closed.
In order to generate braking pressure, the driver applies the brake pedal so that braking pressure develops in the master brake cylinder, which is supplied to the wheel brakes via the open brake line to decelerate the vehicle. Additionally, a device is provided which monitors the rotational behavior of the wheels. This device senses when any one of the wheels tends to lock, prompting the system to switch over to brake slip control. This means that the motor drive will be switched on and that the pump now will be delivering from the storage reservoir into the brake line. The inlet and outlet valves will be actuated so that, for the purpose of decreasing the pressure, the inlet valve will close and the outlet valve will open and to build-up pressure, the inlet valve will open and the outlet valve will close. Thus, for pressure reduction, pressure medium is bled off from the wheel brake and, for pressure increase, pressure medium is supplied from the pump.
As the communication between the master brake cylinder and the pressure modulation valves is not interrupted, the pump will not only deliver to the wheel brakes, but also into the master brake cylinder. As a result, the working pistons will be displaced back into the brake-released position. In the brake's release position, the known central valves will open and further act as control valves so that pressure will remain in the brake line which is proportional to the foot pressure.
This system has the advantage that a hydraulic system for brake slip control is realized by means of a relatively small expenditure with regard to valves. Above all, it is possible to dispense with a separate pressure control valve for controlling the pump pressure and with a separating valve for decoupling the master brake cylinder from the brake lines as described, for instance, in German Published Patent Application (DE-OS) No. 35 42 419.
However, this simple construction also has a disadvantage. In the phases of pressure build-up, the volume requirements of the wheel brakes temporarily exceed the delivery capacity of the pump so that pressure medium is taken out of the master brake cylinder causing the brake pedal to drop a short distance. In the phases of pressure decrease in the wheel brakes, the inlet valve will be closed as explained so that the entire delivery of the pump will be supplied into the master brake cylinder causing the pedal to be reset. The direct coupling of the pump to the master brake cylinder thus has the disadvantage that the pedal will strongly vibrate in correspondence with the control cycles (pressure build-up, pressure decrease). This problem could be solved by dimensioning the pump so that the volume requirements of the wheel brake are made available for control at any time. This, however, involves a considerable increase in cost which is to be avoided at any rate.